In a battery powered mobile unit or device it is necessary to monitor the battery voltage so that the device can be properly shut down before the battery supply voltage drops below a threshold for acceptable operation of the unit. Proper shutdown often involves saving configuration parameters in a non-volatile memory and/or providing either the user or an electrical or mechanical device with proper warning prior to shutdown.
One approach includes simply monitoring the battery voltage. A problem associated with this approach, however, is that the battery voltage, when measured at a point beyond the battery terminals, depends upon the ohmic losses or the drop in voltage caused by a current passing through a resistive path. In addition, the battery itself includes some variable ohmic loss, which is dependent upon the amount of charge remaining in the battery and the current being drawn from the battery. For example, a fully charged battery might measure 4.2 V at the terminals under a very small load such as &lt;1 mA, but measure 3.9 V under a load of 1 A. This voltage drop at the terminals is due to the output impedance of the battery. The output impedance can depend on the charge of the battery. Thus, a half-discharged battery measuring 3.5 V at 1 mA might only provide 3.1 V under a 1 A load. standby and transmit modes with current loads of 10 mA, 30 mA and 700 mA, respectively. The sleep and standby modes may be treated as the same "light" load, and a single threshold voltage can be selected for shutdown under the light load. If the minimum acceptable terminal voltage for the battery is 3.0 V, then one can select a single point under load corresponding to this point. For example, a 3.4 V under the "light" load condition may drop to 3.0 V under the 700 mA load. The mobile phone would then know that at 3.4 V or below, it cannot go to the transmit mode without causing a supply voltage shortage. This simple approach, however, is not well-suited to applications in which there are many different modes of current loading. Because the voltage at the battery terminals can vary significantly among the various loads, selecting a single point under load at which to shut down would not be an efficient use of the battery, as certain applications would be closed prematurely.
Another approach at monitoring the battery charge status includes monitoring the energy flow both into and out of the battery. By monitoring the current into and out of the battery, one can determine its exact charge status. While this approach provides extremely accurate information as to the charge status of the battery, it is very expensive because it requires additional circuitry to monitor the current into and out of the battery.
There is a need, therefore, for a relatively simple but accurate way of monitoring the amount of energy remaining in the battery of an electronic device capable of operating in various modes, each having a particular current load. It would also be desirable to calculate all of the operating mode possibilities available for the electronic device under the present battery condition and to display such options to the user.